(1) Field of the Invention
The present invention relates to a fuel cell in which the solid polymer membrane is used as the electrolyte film.
(2) Related Art
The fuel cell is a cell that generates electricity by supplying hydrogen-rich fuel gas to the anode and oxidizer gas to the cathode and by electrochemically reacting hydrogen and oxygen. As the oxidizer, air is generally used. On the other hand, as the fuel gas, pure hydrogen gas, reformed lower alcohol, or reformed light hydrocarbon such as reformed natural gas and reformed naphtha is used.
There are a variety of fuel cells according to the kinds of the electrolyte, for instance, the phosphoric acid type fuel cell and molten carbonate fuel cell. Especially, the fuel cell in which the solid polymer membrane is used as the electrolyte film has been recently under development.
The basic structure of this type of fuel cell is as follows. A cell is formed by disposing a cathode on one side of a solid polymer membrane and an anode on the other side. The cell is sandwiched between a plate with fuel gas channels and a plate with oxidizer channels. This fuel cell can generate electricity with high performance at a relatively low operating temperature. In a practical fuel cell, such cell units are stacked in layers to obtain high power output.
The dry solid polymer membrane has lower ion conductivity. As a result, moistened fuel gas and oxidizer gas are generally supplied to the fuel cell in order to moisten the solid polymer. Recently, however, the fuel cell to which dry air is supplied has been developed since the demand to make the fuel cell more compact has grown.
In order to generate electricity by this fuel cell with high performance, it is preferable to evenly moisten the solid polymer membrane. In fact, however, the solid polymer membrane is not evenly moistened, i.e., the solid polymer membrane is overly moist or dry in some parts.
More specifically, around the entrance for the oxidizer (around the entrance for air), water tends to evaporate into the oxidizer gas, so that the solid polymer membrane tends to dry. Especially, in the dry-air-type fuel cell, a large amount of water is dissipated from the solid polymer membrane at the entrance of the oxidizer, so that the solid polymer membrane tends to dry in this part.
When flowing through the gas channels, the oxidizer gas absorbs water from the cell. As a result, the humidity of the oxidizer increases and it becomes more difficult to dissipate water into the gas as the oxidizer travels from the entrance through the exit.
If the humidity of the oxidizer is set high, the solid polymer membrane can be moist enough at the entrance for the oxidizer. In this case, however, the solid polymer membrane is overly moist at the exit, so that high electricity generation performance cannot be obtained.
A proposed solution to this problem is Japanese Patent Laid-Open Publication No. 11-154523. This application proposes a cell in a solid polymer electrolyte fuel cell in which gas permeability is lower at the entrance for the gas than at the exit in the gas diffusion layer on the cathode and/or the gas diffusion layer on the anode.
In the solid polymer electrolyte fuel cell, the solid polymer membrane 201 is sandwiched between the cathode 210 and the anode 220 in the cell, and electricity is generated by flowing the oxidizer gas (the arrow 231) cross the cathode 210 and the fuel gas (the arrow 232) across the anode 220 as shown in FIG. 12. The gas diffusivity in the part 211, which is closer to the entrance for the gas, is set to be smaller than that in the part 212, which is closer to the exit for the gas. The gas diffusivity is adjusted by changing the thickness and the ratio of pores of the gas diffusion layer. More specifically, the gas diffusivity is set relatively small by setting the ratio of pores small or the thickness large in the entrance part 211. On the other hand, the gas diffusivity is set relatively large by setting the ratio of pores large or the thickness small in the exit part 212.
As has been described, the solid polymer membrane 201 can be prevented from drying at the entrance for the oxidizer gas by differently adjusting the gas diffusivity in different parts. In this case, however, the oxidizer is not evenly distributed across the cathodes and the concentration polarization is large in the electrode reaction, so that the power output is lowered. This is problematic phenomenon.
It is accordingly the object of the present invention to provide a fuel cell including a cell that is formed by sandwiching a solid polymer membrane between an anode and a cathode that generates electricity with stability and high performance by evenly moistening the solid polymer membrane.
The above-mentioned first object may be achieved by disposing a layer on the cathode catalyst layer so as to face the oxidizer channels, the layer being conductive and gas-permeable, water permeability of which is set to be lower in an area closer to an entrance for the oxidizer than in an area closer to an exit for the oxidizer.
Here, the water permeability indicates the amount of water that moves through a unit area of a layer when the water concentration on one side of the layer is different from the water concentration on the other side. More amount of water moves through the layer, higher the water permeability.
When the layer in which the water permeability has been adjusted is formed between the cathode catalyst layer and the oxidizer channels in this manner, the tendency that the amount of water evaporating from the solid polymer membrane is larger at the entrance for oxidizer gas than at the exit can be repressed. As a result, the solid polymer membrane can be evenly moistened.
The gas diffusivity can be kept even irrespective of the layer in which the water permeability has been adjusted, so that the oxidizer can be evenly spread in the electrode.
As a result, according to the present invention, the ion exchange can be ensured and the amount of oxidizer gas supplied to the catalyst layer can be even in any area in the cell.
The layer adjusting the water permeability may have the function of a gas diffusion layer by being formed of a conductive, porous material that includes a water repellent. In this case a content of the water repellent in the area closer to the entrance for the oxidizer may be set to be higher than a content of the water repellent in the area closer to the exit for the oxidizer.
The water permeability adjusting layer may include: a gas diffusion layer that is formed of a water-repellent, conductive, porous material; and a mixture layer that is disposed between the gas diffusion layer and the cathode catalyst layer, the mixture layer being formed of a carbon material to which a water-repellent material has been added.
In this case, the water permeability may be adjusted by setting the specific surface area of a first carbon material in the area closer to the entrance for the oxidizer smaller than a specific surface area of a second carbon material in the area closer to the exit for the oxidizer, and by adding the water-repellent material to the carbon material so that water repellency of the mixture layer is higher in the area closer to the entrance for the oxidizer than in the area closer to the exit for the oxidizer.
Note that when the mixture layer is formed mainly of a carbon material as has been described, the mixture layer keeps water. As a result, a water-keeping layer is formed between the gas diffusion layer and the catalyst layer. The water-keeping layer can keeps the solid polymer membrane with higher stability.
According to the present invention, the solid polymer membrane can be kept moist at the entrance for oxidizer even if dry oxidizer gas (air) is supplied, so that electric power can be generated with stability.